The present invention relates to an apparatus for determining a state of deterioration of a purifying performance of a purifier for purifying exhaust gas from an internal-combustion engine (hereinafter referred to as an “engine”).
As one of conventional techniques for determining a deterioration state of a purifier for purifying exhaust gas produced by combustion of an air-fuel mixture in an engine, there is, for example, an approach of determining a deterioration state of a purifier by the use of a linear filter. This approach is disclosed in the Japanese Patent Application Unexamined Publication (Kokai) No. 2003-193898. According to this approach, a process with a linear filter (for example, a bandpass filter) is first performed on an output SVO2 of an oxygen density sensor (O2 sensor) disposed downstream of the purifier. The value of the filtered output is used to determine the deterioration of the purifier.
In general, an exhaust gas purifier of an engine reduces NOx in the exhaust gas through an O2 storage effect and oxidizes unburned HC, CO. When the O2 storage effect deteriorates, purification of the exhaust gas becomes insufficient and a variation in the air-fuel ratio on the upstream side of the purifier appears as a variation in the outputs of the O2 sensor. In other words, a fine noise-like vibration becomes apparent in the waveform of the output SVO2 of the O2 sensor as the purifier deteriorates.
The above-referenced approach uses the SVO2 properties caused by deterioration and compares and analyzes the frequency components of the SVO2 applying a fast Fourier transform (FFT) to the SVO2. As a result of such analysis, it is observed that there is a frequency range in which a power spectrum increases as the purifier deteriorates. In other words, if a certain frequency component that correlates with the deterioration can be extracted, deterioration of the purifier can be determined. Based on this observation, the above-referenced approach uses a bandpass filter to extract from the SVO2 only the frequency component that increases as the purifier deteriorates so as to utilize the extracted frequency component as a parameter for determining the deterioration.
However, according to the above-referenced approach, from time to time a new catalyst (hereinafter referred to as a “fresh catalyst”) and a catalyst having a normal purifying performance (hereinafter referred to as a “normal catalyst”) may not be distinguished from a catalyst having an insufficient purifying performance (hereinafter referred to as a “NG catalyst”).
The fresh catalyst characteristically results in SVO2 of a rectangular waveform. The rectangular waveform includes an edge-shaped wave as well as a step wave. It is known that a step-shaped waveform with edges includes various frequency components.
In the above-referenced approach, the filtering process is performed to extract the components associated with the deterioration from the SVO2. In other words, the components that are regarded to represent the characteristics of the fresh catalyst must be removed. However, the rectangular wave that is formed by the various frequency components cannot be completely removed with the bandpass filter. Similarly, any other linear filter, which is designed to provide a bandpass by using a frequency range, cannot remove such rectangular waveform completely. Therefore, the above-referenced approach includes a problem that even when the catalyst is normal a part of the SVO2 waveform may pass through the bandpass filter and output a discrimination parameter.
Thus, there exists a need for a technique for appropriately extracting a rectangle-shaped waveform component that is characteristic to the fresh catalyst. Accordingly, it is an object of the present invention to provide a high precision deterioration determination device for an exhaust gas purifier of an engine.